Optical wavelength division multiplexing systems (WDM systems) are used in optical transmission networks in order to transmit large volumes of data. In a network node or a terminal node of a simple point-to-point connection, an optical filter unit is provided that comprises at least one optical multiplexer unit and one optical demultiplexer unit. The optical multiplexer unit has a specific number n of channel input ports, and feeds optical channel signals fed to the channel input ports to a WDM output port, wherein the optical channel signals can be combined into one WDM signal. The optical demultiplexer unit has a WDM input port to which the WDM signal to be received is fed. The multiplexer unit splits the optical WDM reception signal into the individual channel signals and feeds each of these to an allocated channel output port. Each channel input port of the multiplexer unit can be fed an optical channel signal of a specified optical carrier wavelength, wherein each carrier wavelength of the fed optical signal must correspond to the channel wavelength allocated to the relevant channel input port, so that the relevant optical channel signal can be fed to the WDM output port correctly and with the lowest possible insertion attenuation.
In the same way, a channel signal of a specified optical carrier wavelength in a received optical WDM signal, wherein this carrier wavelength must correspond to a channel wavelength, can be extracted by the demultiplexer unit from the WDM reception signal and fed to the allocated channel output port. The channel wavelengths for the transmission direction and for the reception direction are here selected so that, for a channel i, both for the transmission direction and also for the reception direction, the same optical channel wavelength is used.
Typical optical filter units here usually have a plurality, for example, 40, channel input ports or channel output ports. To allow bidirectional connection on a specified channel, for each an optical transceiver unit is used that generates, from an electrical data signal fed to it, a corresponding optical data signal that is output on an optical output port of the transceiver unit. This optical output port of the transceiver unit is connected by means of an optical waveguide to the associated channel input port of the multiplexer unit of the optical filter unit. For this purpose, the channel input port of the multiplexer unit must be selected that corresponds, with respect to the channel wavelength, to the optical carrier wavelength of the optical signal output by the transceiver unit or, in other words, the carrier wavelength of the optical signal output by the transceiver unit must correspond to the channel wavelength of the selected channel input port of the multiplexer unit.
In the same way, the associated channel output port of the demultiplexer unit to which the relevant channel wavelength is allocated is connected to an input port of the optical transceiver unit that feeds, in the transceiver unit, the received channel signal to a receiving unit of the transceiver unit that typically has a broadband construction, so that it is able to detect each of the possible channel signals. The receiving unit converts the optical channel signal into a corresponding electrical data signal and can simultaneously carry out signal conditioning and optionally also signal processing.
Such optical transceiver units are typically provided with transmitting units that have an optical transmitting module of the desired channel wavelength. Therefore it is necessary to provide, for each channel wavelength, an optical transceiver unit with the desired (fixed) channel wavelength.
Recently, optical transceiver units have been developed with tunable transmission modules. In the case of such transceiver units, the transmitting unit comprising the relevant transmission module can be driven by a controller unit, so that an optical transmission signal with the desired channel wavelength is generated from the electrical data signal fed to the transceiver unit. The controller unit typically feeds to the transmitting unit only channel information. i.e., it is determined which of several possible discrete channel wavelengths should be used for the optical transmission signal to be generated.
With such tunable transceiver units, it is possible to connect a transceiver unit initially to an arbitrary free channel input port of the multiplexer unit of an optical filter unit. Obviously, the receiving unit of the transceiver unit must be connected to the allocated channel output port of the demultiplexer unit to which the same channel wavelength is allocated. In the case of known transceiver modules, information on which channel wavelength should be used for the optical transmission signal is then fed to the optical transmitting unit.
Here, however, there is the risk that the correct information concerning the channel wavelength to be selected is not transmitted to the controller unit of the optical transmitting unit, for example, because, in the case of the installation of the transceiver unit, the person performing this installation has transmitted incorrect information to the controller unit. In addition to this risk of incorrect installation of the transceiver unit, for example, on an existing WDM system, the requirement of the correct selection of the channel wavelength for a tunable optical transceiver unit also produces a corresponding expense. This is because the person who replaces a transceiver card or who expands the existing system through the addition of one or more additional transceiver cards, must be informed before the installation on which channel wavelengths are already occupied and which channel wavelengths of the system are still available or on which channel wavelengths the transceiver unit to be replaced has transmitted.
If the channel wavelength of a tunable optical transceiver unit is not selected correctly, then the multiplexer unit obviously cannot integrate the optical signal that is fed on a channel input port and that does not have the correct channel wavelength into the WDM signal to be transmitted.